vi PHYLUM NEMATHELMINTHES '297 



AFFINITIES AND MUTUAL RELATIONSHIPS OF THE 



NEMATHELMINTHES. 



The affinities of all the classes of Nemathelminthes are very 

 obscure, and the propriety of grouping them into a single phylum 

 is extremely doubtful. They all agree in being elongated, cylin- 

 drical worms with a ccelome ; there is a certain resemblance 

 between Nematoda and Chsetognatha in the muscular system ; 

 and the lemnisci of Acanthocephala have been compared with the 

 cervical glands of Nematoda. Beyond these points there is little 

 to unite the three classes, and, on the other hand, the proboscis of 

 Acanthocephala recalls the rostellum of Cestoda. 



The three families placed as an Appendix to the phylum present 

 some undoubted resemblance to the Nematoda : this is especially 

 the case in the reproductive organs of the Chsetosomidse, and still 

 more in those of Desmoscolex. But the segmentation of the body 

 in both Desmoscolecida3 and Echinoderida? and the presence of 

 seta? show a certain resemblance to higher worms or Annulata, 

 which will be more fully appreciated when that phylum has been 

 studied. 



